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Kokoro resembles Kotonoha and has the same purple eyes that she has. Unlike her sister though, she has chest-length black hair with small amounts of it pulled up into a ponytail on each side of her head with a single bead.

Unlike Kotonoha, Kokoro is outgoing, energetic, and very talkative. She can make friends easily and may annoy her sister due to being nosy or pushy, but she shows great concern for how unsocial she is. She had no problem telling off Sekai for seducing Makoto when they came to check on Kotonoha.

Kokoro was first seen when she embraces her sister after the latter cames home with Makoto. She realizes that Makoto is Kotonoha boyfriend she started to call him "Makoto-kun", until Kotonoha tells her that she can't do that. So she settles for "Onii-chan", which also annoys her and she forces Kokoro to stop bothering him. She then decides to have Kokoro show Makoto around while she makes tea.

Meanwhile Kokoro tells Makoto to make a promise that he'll stay as her sisters boyfriend. He agrees and they make a pinky swear, causing her to jump on him again - annoying Kotonoha. She grabs Makoto and brings him to her bedroom.

At the summer vacation Makoto visits Kotonoha in her place and Kokoro is seen clinging on him again and asked him if he came to play with her. Makoto informs her that he had come to speak with Kotonoha though, and asks for some privacy.

At some point, Kokoro was hanging out with her friends in town when she witnesses Kotonoha seperating from Makoto and Sekai. She is called by her friends that they are ready to leave, but witnesses the two of them act oddly close, so she relies this information to Kotonoha when she gets home. However, Kotonoha denies it and tells her to stop saying this, thanking her for her concern.

Kokoro shouts at Sekai

The day after the Festival, Kotonoha didn't go to school so Sekai and Makoto come by to visit her. Kokoro realizes Sekai is to blame and quickly starts to berate her upon realizing she was standing behind Makoto. She accuses Sekai for being the reason behind Kotonoha's sudden condition but is stopped once Kotonoha arrives. She kicks the two of them off of the property and shuts the door.

Kokoro pays Kotonoha a visit in her bedroom to see how she is. She watches them leave before Kokoro asks how she is, then claims that she is fine. Seeing no problem with it, Kokoro takes off in silence.

A week passes and Makoto continues to heal in the Hospital. As Sekai shows up, having finished with her errand, he gets an alarmed call from Kokoro telling him that Kotonoha is covered in blood and isn't moving. He panics and realizes that Sekai must have done something, and Sekai reveals that she stopped by Kotonoha's house and snuck inside, then stabbed Kotonoha and took off.

Kokoro and Makoto gets along with each other intially when Kotonoha invites him to his house and even makes a pinky swear with him to be her sister's boyfriend forever. She soon grows a bit hestile towards him as he tries to defend Sekai from her, but she still views him as a brotherly figure and called him in the Hospital to inform him of Kotonoha being stabbed.

Kokoro and Kotonoha get well together but sometimes end up in typical sisterly disputes. Kokoro consistantly shows concern for her older sister, due to being unsociable and because her mood worsens again.